1. Field of the Invention
The present invention relates to a current detecting apparatus for detecting a current flowing in a resistance welding machine.
2. Description of the Related Art
In resistance welding, a welding current is a most significant welding condition and must properly be controlled and monitored. Up until now, a resistance welding control unit or a monitoring unit has often employed a toroidal coil to detect a current flowing in a resistance welding machine. In general, the toroidal coil is mounted in such a manner as to surround a conductor through which a current flows in the resistance welding machine. When the current flows, a magnetic flux generated around the conductor passes through a toroidal coil conductor to produce an induced voltage corresponding to a time differential of the magnetic flux (accordingly, a time differential of the current).
In this manner, a signal issued from the toroidal coil represents a differential waveform of a current flowing through the resistance welding machine and hence it is not adapted to current measurements (mean value calculation or effective value calculation) unless it is modified. Thus, a waveform restoring unit comprising an integrating circuit is provided to integrate the output signal from the toroidal coil to thereby produce an integral value signal (current waveform restoration signal). In general, such an integrating circuit includes an operational amplifier.
As described above, when a current flows in the resistance welding machine during the welding operation, the toroidal coil outputs a signal indicative of a differential waveform of the current, and an integral value signal indicative of a current waveform is derived from the waveform restoring circuit on the basis of the differential waveform signal. Accordingly, in cases where a current flows in a waveform as shown in FIG. 9(A), the waveform restoring unit provides as its output a signal having a waveform similar to this current waveform.
However, actually the waveform restoring unit suffers from a disadvantage that, due to the zero-point fluctuations or the like of the operational amplifier constituting the integrating circuit, the center level (zero-point level) of the output signals, namely, of the integration value signals may deviate from the reference value (usually, 0 volt) with the elapse of time as shown in FIGS. 9(B) and 9(C).
This problem conspicuously arises in seam welding in particular. The seam welding is a process in which a joint or a seam of a relatively thin metal plate is continuously or intermittently formed by welding in a threading manner, and which requires an incomparably long current supply time (e.g., several tens of seconds) for a single welding operation as compared with that of the spot welding. For this reason, even though the zero-point level of the integration value signals remains stably in the vicinity of the reference value immediately after the start of supply of welding current, an error may gradually occur according as the current supply time elapses, resulting in a lowered accuracy of the current detection values.
However, the conventional resistance welding monitoring unit makes a monitor judgment on the basis of such less reliable current detection values, so that the accuracy or the reliability of the monitor judgment was low. In the case of performing a feedback constant current control, it was also difficult for the conventional resistance welding control unit to correctly conform the current value to a set value since the errors of the current detection values reflect on the control values.